The present invention relates to an apparatus and method for plasma etching. More particularly, it relates to an apparatus and method for plasma etching wherein a target film is etched by a plasma generated with a high-frequency induction field produced by a spiral coil disposed in opposing relationship with a sample stage provided in a chamber.
With the increasing miniaturization of a semiconductor integrated circuit element in recent years, exposing light with a shorter wavelength has been used in a lithographic step. At present, the use of a KrF excimer laser (with a wavelength of 248 nm) or an ArF excimer laser (with a wavelength of 193 nm) is becoming prevalent.
As the wavelength of exposing light becomes shorter, the reflectivity of light reflected from a substrate after exposing a resist film becomes higher so that the reflected light from the substrate is more likely to cause variations in the size of a resist pattern. To prevent the reflected light from being incident on the resist film, there has recently been used a process wherein an organic bottom anti-reflective coating (hereinafter referred to as ARC) is formed under the resist film. The ARC process is primarily used in the manufacturing of a semiconductor element in a high-performance device with design rules whereby a gate width is 0.25 xcexcm or less.
In the ARC process, it is necessary to etch the ARC after a resist pattern is formed by a conventional lithographic technique. Of a variety of plasma etching apparatus used to etch the ARC, an apparatus for inductively coupled plasma (ICP) etching with a spiral coil is used frequently.
As examples of the inductively coupled apparatus for plasma etching, an apparatus for inductively coupled plasma etching having a planar coil (see U.S. Pat. No. 4,948,458), an apparatus for inductively coupled plasma etching with a dome-shaped coil (see U.S. Pat. No. 5,614,055), and the like are known.
Referring to FIG. 10, a conventional apparatus for inductively coupled plasma etching having a planar single spiral coil will be described.
As shown in FIG. 10, a sample stage 2 as a lower electrode to which high-frequency power is applied is disposed in the lower portion of a grounded chamber 1 having an inner wall covered with an insulator such as ceramic, alumina, or quartz. A semiconductor substrate 3 as a sample to be etched is placed on the sample stage 2. The chamber 1 is provided with gas inlet ports (not shown) for introducing etching gas into the chamber 1 via a mass flow controller and with a gas outlet port 5 connected to a turbo pump for setting pressure in the chamber 1 to the order of 0.1 Pa to 10 Pa.
A single spiral coil 4 of inductively-coupled type is disposed atop the chamber 1 externally thereof in opposing relationship with the sample stage 2. The single spiral coil 4 has one end connected to a high-frequency power source via a matching circuit (not shown) and the other end connected to a wall of the chamber 1 and thereby grounded. The arrangement allows the single spiral coil 4 to generate a high-frequency induction field so that etching gas introduced into the chamber 1 is changed into a plasma. The etching gas changed into the plasma is guided by high-frequency power applied to the sample stage 2 toward the target film on the semiconductor substrate 3 held by the sample stage 2 so as to etch the target film.
When the present inventors performed an etching process. with respect to an ARC as the target film by using a plurality of inductively coupled apparatus for plasma etching each having the planar single spiral coil 4 mentioned above, the problem occurred that the inplane uniformity of the etching rate varied with the different apparatus for plasma etching, though they were of the same model.
The inplane uniformity of the etching rate is defined as the degree of variations in etching rate across the surface of the target film and expressed as 3"sgr"/xcexcxc3x97100 (%), where "sgr" is the standard deviation of a data value and xcexc is the mean value of the data value. When variations in data value exhibit a normal distribution, 3"sgr" represents a deviation including 99.74% of the data value. The following equation (1) shows 3"sgr" and xcexc specifically.                               3          ⁢          σ                =                  3          ⁢                                                                      ∑                                      i                    =                    1                                    n                                ⁢                                  xe2x80x83                                ⁢                                                      (                                          Xi                      -                      μ                                        )                                    2                                            n                                                          Equation        ⁢                  xe2x80x83                ⁢        1                        where      ⁢              xe2x80x83            ⁢      μ      ⁢              :              ⁢      mean      ⁢              xe2x80x83            ⁢      value        =                            ∑                      i            =            1                    n                ⁢                  xe2x80x83                ⁢        Xi            n            n    ⁢          :           ⁢    number    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    samples        Xi    ⁢          :           ⁢    i    ⁢          -        ⁢    th    ⁢          xe2x80x83        ⁢    data    ⁢          xe2x80x83        ⁢          value      .      
Conditions for the plasma etching process when the inplane uniformity of the etching rate was measured by using the conventional apparatus for plasma etching are as shown in Table 1.
In Table 1, ICP denotes high-frequency power applied to the single spiral coil 4 and RF denotes high-frequency power applied to the sample stage 2.
The models of apparatus for inductively coupled plasma etching and the inplane uniformities of the respective etching rates are as shown in Table 2. As shown in FIG. 18(a), etching was performed with respect to the ARC 11 as a target film formed on the semiconductor substrate 10.
As will be understood from Table 2, the inplane uniformities of the etching rates for the ARC 11 were xc2x14.5% for the apparatus A, xc2x12.1% for the apparatus B, xc2x15.6% for the apparatus C, xc2x15.1% for the apparatus D, xc2x13.3% for the apparatus E, xc2x16.8% for the apparatus F, and xc2x12.6% for the apparatus G and not constant.
The etching rate which is inferior in inplane uniformity causes variations in the actual amount of etching across the surface of the target film. If the actual amount of etching varies across the surface of the target film, an adverse effect is produced such as variations in the characteristics of a FET in the case of forming the gate electrode of the FET by etching.
In view of the foregoing, it is therefore an object of the present invention to improve the inplane uniformity of an etching rate when etching is performed with respect to a target film by using an apparatus for plasma etching and to reduce variations in the inplane uniformity of the etching rate from apparatus to apparatus for plasma etching of the same model.
As a result of various examinations of the above apparatus A to G, the present inventors found that the positional relationship between the single spiral coil 4 and the gas outlet port 5 varied with the different models of apparatus.
Assuming that the positional relationship between the single spiral coil 4 and the gas outlet port 5 affects the uniformity of the etching rate, the present inventors examined the varying positional relationship between the single spiral coil 4 and the gas outlet port 5 and found that the varying positional relationship between the single spiral coil 4 and the gas outlet port 5 caused uneven distribution of reactive radicals over the plasma generation region in the chamber 1. A description will be given below to the finding.
First, as shown in FIG. 11, it is assumed that the single spiral coil 4 consists of three portions which are: a coil portion 4a contributing directly to the generation of a high-frequency induction field; a power-source-side withdrawn portion 4b positioned between the coil portion 4a and the high-frequency power source 6, and a ground-side withdrawn portion 4c positioned between the coil portion 4a and a ground source 7. It is also assumed that the coil portion 4a consists of two regions separated by a first line L1 linking a power-source connection point A between the coil portion 4a and the power-source-side withdrawn portion 4b to the center point B of the coil portion 4a. Of the two regions of the coil portion 4a separated by the first line L1, the region containing a portion connected directly to the power-source connection point A is defined as a higher-voltage region (region where high-frequency voltage is relatively high) and the region not containing the portion connected directly to the power-source connection point A is defined as a lower-voltage region (region where high-frequency voltage is relatively low).
On the other hand, it is assumed that the sample stage 2 consists of two regions separated by a second line L2 perpendicular to a line linking the center portion C of the sample stage 2 to the center portion D of the gas outlet port 5, as shown in FIG. 12. Of the two separate regions, the region closer to the gas outlet port 5 is defined as an exhaust-side region and the region further away from the gas outlet port 5 is defined as a counter-exhaust-side region.
As shown in FIG. 13, an angle formed between the line 2 and the line L1 which is rotating clockwise relative to the line L2 starting from the state in which the exhaust-side region coincides with the lower-voltage region in overlapping relation and the counter-exhaust-side region and the higher-voltage region are in overlapping relation is defined as a rotation angle xcex8.
The rotation angle xcex8 for each of the foregoing apparatus A to G is as shown in Table 3.
As will be understood from Table 3, the rotation angle xcex8 formed between the second line L2 and the first line L1 which is rotating clockwise with respect to the second line L2 varies with the conventional apparatus for plasma etching. From a comparison between Tables 2 and 3, it will also be understood that there is the correlation between the rotation angle xcex8 and the inplane uniformity of the etching rate and that the inplane uniformity of the etching rate improves as the rotation angle xcex8 increases.
The reason for this may be that the quantity of reactive radicals is larger in the area of the plasma generation region in the chamber 1 corresponding to the counter-exhaust-side region of the sample stage 2 than in the area thereof corresponding to the exhaust-side region of the sample stage 2, as shown in FIG. 14(a), and that the quantity of reactive radicals is larger in the area of the plasma generation region in the chamber 1 corresponding to the higher-voltage region of the single spiral coil 4 than in the area thereof corresponding to the lower-voltage region of the single spiral coil 4, as shown in FIG. 14(b).
Accordingly, if the exhaust-side region of the sample stage 2 and the higher-voltage region of the single spiral coil 4 are positioned on the same side and if the counter-exhaust-side region of the sample stage 2 and the lower-voltage region of the single spiral coil 4 are positioned on the same side, reactive radicals are evenly distributed over the plasma generation region in the chamber 1, as shown in FIG. 15(a). Conversely, if the exhaust-side region of the sample stage 2 and the lower-voltage region of the single spiral coil 4 are positioned on the same side and if the counter-exhaust-side region of the sample stage 2 and the higher-voltage region of the single spiral coil 4 are positioned on the same side, reactive radicals are unevenly distributed over the plasma generation region in the chamber 1, with reactive radicals present in smaller quantity in the area of the plasma generation region in the chamber 1 corresponding to the exhaust-side region of the sample stage 2 and to the lower-voltage region of the single spiral coil 4 than in the area thereof corresponding to the counter-exhaust-side region of the sample stage 2 and to the higher-voltage region of the single spiral coil 4.
The present invention has been achieved based on the foregoing findings and intends to evenly distribute reactive radicals over the plasma generation region in the chamber 1 by positioning the higher-voltage region of the spiral coil and the exhaust-side region of the sample stage on the same side relative to the center axis of the chamber. While the conventional apparatus for plasma etching attributes importance only to applying a current of ions having a uniform density to the semiconductor substrate 3 placed on the sample stage 2, the present invention attributes importance to supplying uniform reactive radicals to the semiconductor substrate 3 placed on the sample stage 2.
Specifically, an apparatus for plasma etching according to the present invention comprises: a chamber; a gas inlet port provided in the chamber to introduce etching gas into the chamber; a gas outlet port provided in a side portion of the chamber to exhaust the gas from the chamber; a sample stage provided within the chamber; and a spiral coil disposed externally of the chamber and in opposing relationship with the sample stage to generate a plasma composed of the etching gas with a high-frequency induction field, a higher-voltage region of the spiral coil and an exhaust-side region of the sample stage being positioned on substantially the same side relative to a center axis of the chamber.
When the higher-voltage region of the spiral coil and the exhaust-side region of the sample stage are positioned on substantially the same side, there are cases where the rotation angle xcex8 formed between the second line L2 and the first line L1 which is rotating clockwise relative to the second line L2 is in the range of +135xc2x0 to +180xc2x0, i.e., the overlapping angle is in the range of 135xc2x0 to 180xc2x0, as shown in FIG. 16(a), or where the rotation angle xcex8 formed between the second line L2 and the first line L1 which is rotating clockwise relative to the second line L2 is in the range of xe2x88x92135xc2x0 to xe2x88x92180xc2x0, i.e., the overlapping angle is in the range of 135xc2x0 to 180xc2x0, as shown in FIG. 16(b).
In the apparatus for plasma etching according to the present embodiment, the higher-voltage region of the spiral coil and the exhaust-side region of the sample stage are positioned on substantially the same side relative to the center axis of the chamber so that the lower-voltage region of the spiral coil and the counter-exhaust-side region of the sample stage are inevitably positioned on substantially the same side relative to the center axis of the chamber. Since the quantity of reactive radicals is relatively small in the area of the plasma generation region in the chamber corresponding to the lower-voltage region of the spiral coil and relatively large in the area thereof corresponding to the counter-exhaust-side region of the sample stage, the quantity of the reactive ions is averaged in the area of the plasma generation region in the chamber corresponding to the lower-voltage region and to the counter-exhaust-side region of the sample stage. On the other hand, since the quantity of reactive radicals is relatively large in the area of the plasma generation region in the chamber corresponding to the higher-voltage region of the spiral coil and relatively small in the area thereof corresponding to the exhaust-side region of the sample stage, the quantity of the reactive ions is averaged in the area of the plasma generation region in the chamber corresponding to the higher-voltage region and to the exhaust-side region of the sample stage.
Thus, the quantity of reactive radicals is averaged in each of the area of the plasma generation region in the chamber corresponding to the higher-voltage region and to the exhaust-side region and the area thereof corresponding to the lower-voltage region and to the counter-exhaust-side region, resulting in reactive radicals evenly distributed over the plasma generation region in the chamber.
In the apparatus for plasma etching according to the present invention, the spiral coil is preferably a single spiral coil having a planar configuration or a domed configuration.
In the apparatus for plasma etching according to the present invention, the spiral coil is preferably the longest one of a plurality of spiral coils arranged in parallel with each other.
In the apparatus for plasma etching according to the present invention, high-frequency power applied to the spiral coil is preferably higher than high-frequency power applied to the sample stage.
A method for plasma etching according to the present invention comprises: a plasma generating step of changing etching gas introduced into a chamber into a plasma with a high-frequency induction field generated by a spiral coil disposed in opposing relation with a sample stage in the chamber; an etching step of guiding the plasma toward a target film on a substrate held by the sample stage to etch the target film; and a gas exhaust step of exhausting gas from the chamber through a gas outlet port provided in a side portion of the chamber, the etching step including the step of guiding the plasma toward the target film to etch the target film with a higher-voltage region of the spiral coil and an exhaust-side region of the sample stage being positioned on substantially the same side relative to a center axis of the chamber.
In accordance with the method for plasma etching according to the present invention, the quantity of reactive radicals is averaged in each of the area of the plasma generation region in the chamber corresponding to the higher-voltage region and to the exhaust-side region and the area thereof corresponding to the lower-voltage region and to the counter-exhaust-side region, resulting in reactive radicals evenly distributed over the plasma generation region in the chamber.
In the method for plasma etching according to the present invention, high-frequency power applied to the spiral coil is preferably higher than high-frequency power applied to the sample stage.
In the method for plasma etching according to the present invention, the target film is preferably an organic film.
In the method for plasma etching according to the present invention, the target film is preferably an organic bottom anti-reflective coating or a resist film.
Thus, in the apparatus for plasma etching or method for plasma etching according to the present invention, the quantity of reactive radicals is averaged in each of the area of the plasma generation region in the chamber corresponding to the higher-voltage region and to the exhaust-side region and the area thereof corresponding to the lower-voltage region and to the counter-exhaust-side region, so that reactive radicals are evenly distributed over the plasma generation region in the chamber. Consequently, the etching rate for the target film on the substrate held by the sample stage becomes uniform across the surface of the target film, while the inplane uniformity of the etching rate does not vary with different apparatus for plasma etching.